More specifically, "break down" = verb, and gets conjugated as "breaks down", "breaking down", etc., thus the logic for writing it as two words (as opposed to "breakdown", which would be pluralized as "breakdowns").
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KosmonautDec 12 '10 at 22:42

Is the report detailing expenditure for a particular month or, more generally, monthly? I list here a bunch of possible solutions:

"I prepared a detailed report on my/our monthly expenditure."

"I prepared a report detailing my/our monthly expenditure."

"I prepared a report detailing my/our expenditure for the month."

The above examples are probably the most succinct way to express your idea. I omitted "total" because I think it could be superfluous but you could retain it, if you like. Also, "expenditure" is the word to use here, not "spending". I included a qualifying pronoun but you could leave this out if the context of the sentence already suggests whose expenditure it is. If you really need to mention the "breakdown" aspect, then here are a few more solutions:

"I prepared a report on my/our monthly expenditure with an itemized breakdown."

"The monthly report I prepared broke down our expenses by item/item by item."

"I prepared a report with a breakdown of my/our monthly expenditure."

"I prepared a report breaking down our expenses for the month."

Of course, all these solutions emphasize different aspects of your statement. There are probably a hundred and one ways of saying what you want to say. I also think "breakdown by items" is tautological. "Breakdown" (a one-word noun, by the way) already implies a component-by-component analysis.

There is also the technical term "breakdown" (in the context of electronics) which is also written as a single word.
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user730Dec 14 '10 at 0:14

True. Indeed, there are a host of other meanings, as well: collapse, deterioration (of health), failure (mechanical or otherwise), decomposition (chemical or physical). I restricted my definition to the context of the author's example.
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Jimi OkeDec 14 '10 at 0:20